When an electrophotographic printer outputs an image whose density is, for example, high on the far side in a direction perpendicular to the paper convey direction and low on the near side, the toner consumption amount changes between the far and near portions of the developing unit, as shown in FIG. 1. That is, the toner consumption amount is large on the far portion in the developing unit and small on the near portion. Especially when print jobs of images whose density is biased continue, the difference in toner consumption amount becomes large. At this time, if a sensor which detects the toner amount is arranged on the near side in the developing unit, it determines that toner remains, though the remaining toner is running short on the far side. Thus, the developing unit is not replenished with toner, no sufficient amount of toner can be supplied to an area of print paper that corresponds to the far side, and no expected density can be attained. To the contrary, if the sensor is arranged on the far side in the developing unit, it determines that no toner remains. In this case, the developing unit is replenished with toner, and toner may be excessive on the near side where the consumption amount is large. Excessive toner may cause contamination by toner scattering, and so-called toner fog on print paper.
When the toner amount on print paper is biased, the fixing unit may also pose a problem. For example, when the toner amount is large on the far side on a toner image on print paper, the fixing unit is deprived of a larger amount of heat on the far side in the fixing unit. The temperature decreases on the far side in the fixing unit, and toner on the far side of print paper does not satisfactorily fuse, resulting in a fixing error. If the fixing temperature is increased to prevent the temperature drop, the temperature on the near side in the fixing unit also rises, and the fixing unit degrades, also causing a fixing error.
Further, the photoconductor drum may also generate a problem. For example, when images whose amount of toner application is biased are successively formed, a memory phenomenon occurs on the photoconductor drum, and a ghost caused by the memory phenomenon may appear in a subsequent print job. Note that the memory phenomenon means a state in which the electrical current potential of the photoconductor drum cannot be reset to the initial value and a previous latent image remains.
A problem may occur even when print jobs of images in which the density is different between the leading and trailing edges of print paper in the paper convey direction continue. Particularly when the density at the leading edge is high, separation between the fixing unit and print paper may fail after toner fixing, and print paper may be wound around the fixing unit.
In this manner, an error is highly likely to occur in successive print jobs of images in which the density is different between the far and near sides and between the leading and trailing edges in the paper convey direction. Especially in variable print output, identical background images are successively output, so the above-described errors occur at higher possibility.